As can be gathered inter alia from the aforementioned applications and patents, one of the principal ways of insuring satisfactory removal of sulfur and sulfur compounds from industrial flue gases produced by the combustion of fossil fuels, is the scrubbing of such gases with water containing such calcium compounds.
In scrubbers operating in accordance with these principles, the scrubbing solution and suspension is recirculated, i.e. the sump product of the column is collected and recycled to the head of the column so that within the column the descending scrubbing liquid passes in counterflow to the rising flue gases.
In the sump or, stated equivalently, at a lower portion of this column, an oxidation is carried out using air or another oxygen-containing gas so that any calcium sulfite or calcium hydrogen sulfite (calcium bisulfite) is oxidized to calcium sulfate which can be recovered from the scrubbing liquid circulation.
This means that a scrubber will be provided with a pump recirculating the scrubbing liquid from the sump to the head or upper portion of the column, with auxiliary means for the introduction of air, with means for introducing calcium carbonate as the preferred calcium compound into the scrubbing liquid, and means for withdrawing material from the sump product or the sump product itself.
The flue gas to be treated may have previously been cooled in a heat exchanger before passage through the scrubber and may pass through the heat exchanger again after scrubbing.
An apparatus working in accordance with these principles has been illustrated and described in VGB-Kraftwerkstechnik, 1983, pages 335-344, FIGS. 8, 9 and 10. This system operates quite satisfactorily and represents an efficient system for the desulfurization of flue gas although the efficiency of its operation has been found to vary with different operating conditions. Furthermore it is always desirable to increase the efficiency of such a system.